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Over the past month, we’ve discussed human remains that were centuries—King Richard III—if not a millennium old—King Alfred the Great. For remains of this age, classic carbon dating is the most reliable way of determining time since death. But is there a more precise way to date more recent remains, remains that might only be thirty to fifty years old, instead of six hundred? There is, and that method uses the fallout from nuclear testing following the Second World War to determine time since death.

Following the end of the Second World War, nuclear weapons were tested by the United States, the United Kingdom and Russia. The fallout from this testing radically changed the percentage of radioactive carbon—14C—in the atmosphere, spiking significantly in the early 1960’s before peaking in 1963 at a level nearly twice that of 1950. Atmospheric 14C levels fell slowly in the decades following, but still remain 15% higher than in 1950.

Average atmospheric 14CO2 for the northern hemisphere

Just as strontium is incorporated into living organisms, 14C in atmospheric CO2 enters the food chain when plants use it to manufacture carbohydrates and proteins during photosynthesis. Those plants are then eaten by herbivores and become a permanent part of that animal’s bone structure. As a result, 14C from samples taken from skeletal remains after the 1950’s can be compared to the bomb curve to determine relevant dates. Samples taken from the mid-shaft of long bones represent childhood 14C levels. Spongy cancellous bone sampled from the ends of long bones will show a greater amount of turnover and remodeling that correlates closely to the date of death. Enamel from teeth captures a snapshot of the time when the tooth developed and erupted. If all the values fall in the pre-1950’s range, a different manner of aging the remains must to be used. But for those values that fall post 1950, a window of only a few years can be determined for the date of death.

The slow drop in atmospheric 14CO2 following the early 1960s is due to the signing of the Limited Test Ban Treaty. In August of 1963, representatives from the United States, Russia and the United Kingdom signed a treaty banning all nuclear testing in the atmosphere, in space or under water. In the decades that followed, 123 additional countries signed the ban (the most recent was Montenegro in 2006), leaving 58 states as non-signatory.